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Gatehouseite occurs as radiating or divergent groups of bladelike crystals up to 100 μm by 20 μm by 5 μm in size and as overgrowths on arsenoclasite that are up to 5 mm long.[5] The transparent mineral can be brownish-orange or yellow in color.[3] Gatehouseite is the phosphorus analogue of arsenoclasite.[5]

In May 1987, Glyn Francis, an employee at the Iron Monarch mine in South Australia, submitted for identification some mineral specimens from the mine to A. Pring. Very small, unidentified pale brownish orange crystals were seen in one specimen; study showed they had a formula of the type M5(XO4)2(OH)4 and contained manganese, phosphorus, and a minor quantity of arsenic. The powder X-ray diffraction pattern could not be recorded as insufficient material was available. Another specimen in the same group consisted of arsenoclasite crystals overgrown by what appeared to be its as yet undescribed phosphorus analogue.[4] Francis later discovered more of the brownish orange crystals in sufficient quantity to obtain an X-ray diffraction pattern. This proved that the crystals and the overgrowths were the same, new mineral species.[5]

Due to the small size and intergrowth of the crystals, single-crystal X-ray crystallography is difficult to perform on gatehouseite. In 2011, this technique was successfully used to determine the crystal structure of the mineral.[2]

Gatehouseite has the space group P212121. The crystal structure consists of Mn(O, OH)6octahedra and PO4tetrahedra. The five manganese sites are occupied by manganese and small amounts of magnesium. The two phosphorus sites are occupied by phosphorus and small amounts of silicon and arsenic.[2]